Thermal transfer printers are well known in the art. In such printers, a transfer ribbon coated on one side with a heat-transferable ink layer is interposed between the surface of a non-sensitized web and a thermal print head having a line of small heater elements. When an electrical signal or pulse is applied to a selected subset of the heater elements, localized melting and transfer of the ink to the web occurs, resulting in a corresponding line of dots being transferred. The web is then advanced to print an adjacent location, and the transfer ribbon is repositioned to provide a replenished ink coating. The selecting and heating process is repeated to print an adjacent line of dots. Patterns of successive dots produce printed text or graphics on the web.
Thermal transfer printers are particularly well suited to printing a web of individual tags, tickets, and labels. In such printers, the web is advanced past the print head so the trailing edge of a tag, ticket, or label extends beyond a mechanical cutter or tear-off edge. To reduce waste, it is preferable to backfeed the web before printing again. This results in a corresponding backfeed of the transfer ribbon and the potential for slack ribbon. If slack ribbon is allowed to remain, the resulting loss of ribbon tension may cause the ribbon to wrinkle upon advance, with a resulting loss of print quality. To prevent this, known thermal label ticket and tag printers are typically equipped with a spindle for the ribbon supply roll having a torsion spring and clutch. Forward advancement of the ribbon winds the spring until the spring force overcomes the clutch force, at which point the ribbon feeds at a desired tension determined by the clutch torque.
If a thermal transfer printer according to the prior art must further be capable of printing a range of label, ticket, or tag widths, then the ribbon tension will vary with ribbon width, and no single setting of clutch torque may suffice. The ribbon may wrinkle if the tension is set too low to accommodate a narrower ribbon than is being used, and it may slip if it is set too high to accommodate a wider ribbon than is being used. To address this, the ribbon supply spindle may be equipped with user or technician adjustment for spindle torque. Alternatively, the ribbon supply spindle can be segmented and may have a separate spring and clutch for each segment, such that wider ribbons will engage progressively higher torque segments to maintain relatively constant ribbon tension. Such measures and the cost of frictional components that will last the life of the printer contribute significantly to the cost of the printer.
European Patent Application 0 408 356 A2 to Inoue teaches a ribbon supply core that is reversely rotated to prevent slack. However, the mechanism is reverse driven by the printer rather than operating on elastically stored energy, and it is connected to the printer frame rather than being part of a ribbon cartridge.
U.S. Pat. No. 5,284,396 to Masamura, et. al. teaches two embodiments of a ribbon supply spindle in which energy is stored in an extension spring or torsion spring. In the preferred embodiment, energy is stored in an extension spring and the resulting torque is transferred to a rotatable shaft on which a ribbon supply spool is fixedly mounted. In the alternate embodiment, the shaft is fixed and a ribbon supply spool is rotatably mounted and made nonremovable by a collar, which bears upon a clutch plate and torsion spring, which provides the stored energy. In both embodiments, however, the spring and clutch are part of the printer mechanism, and therefore require operator torque adjustment to accommodate a range of ribbon widths. Moreover, such components must be made of material suitable to the useful life of the printer rather than the useful life of the ribbon.
European Patent Application 0 165 396 to Kitagishi teaches a ribbon cassette with a constant tension imparting mechanism consisting of friction members that are compressed by a plate spring and which sandwich the ribbon. The spring provides frictional force rather than storing energy, and is “H-shaped” to specifically prevent it from doing so, thereby producing equal drag in both directions of ribbon movement. Thus, if the ribbon according to Kitagishi is advanced then released with slack, the slack will remain.
U.S. Pat. Nos. 6,126,344 and 5,788,387 and 5,595,447 to Takayama et al. teach a tape cartridge and printing device having an anti-slack mechanism for preventing slack of the ink ribbon through engagement of a ribbon winding core with an anti-rotational engagement piece. This mechanism, however is intentionally disengaged when the cartridge is set in the printing device rather than being intended to work during printing.
U.S. Pat. No. 4,838,716 to Shinada teaches a ribbon cartridge having a brake mechanism for preventing unnecessary rotation of the feeding ribbon roll. A spring is used to urge a takeup roller against a driven roller so as to pull ribbon from the supply. However, Shinada does not teach or suggest a mechanism to store energy in the supply cartridge or roll and retract slack ribbon into the cartridge or onto the roll.